Pressurized-water nuclear reactors comprise a vessel of generally cylindrical shape containing the core of the reactor and arranged with its axis vertical in a cylindrical vessel well having a lower bottom aligned with the vessel. The core of the nuclear reactor is cooled by pressurized water circulating in the primary circuit of the reactor and on the inside of the vessel in contact with the fuel assemblies.
In some accidents occurring in the nuclear reactor and resulting in a loss of the core-cooling function, it is necessary, in view of the very serious potential consequences, and despite the remote likelihood of such an event, to consider the possibility that the emergency injection circuits of the reactor could not be put into operation. Such a failure could precipitate a sequence leading to meltdown of the core in the absence of cooling water, followed by destruction of the vessel bottom by break-through and the flow of the mass of melting core and the materials surrounding the core into the concrete well holding the reactor vessel. Contact between the molten mass of fuel and materials surrounding the fuel, called corium, the temperature of which can reach 2500 to 2800.degree. C., and the bottom of the concrete vessel well in the absence of cooling can bring about the complete destruction of the bottom of the well. The corium can then penetrate into the raft of the containment of the reactor, destroy this raft and contaminate the groundwater tables present in the ground at the site of the nuclear reactor. The advance of the corium into the ground can stop only when the residual power of the corium has decreased to a sufficient extent.
Various devices for preventing contact between the corium and the bottom of the concrete vessel well have been proposed.
The known devices generally make it possible to spread the mass of corium over a particular area, so that the power to be dissipated per unit area is as low as possible and is compatible with the possibilities for cooling by fluids. It has been proposed, for example, to recover and contain the corium in a metal pouch lined internally with refractory materials, partial fusion of which absorbs energy in a transient manner and makes available a sufficient length of time in which to submerge the metal pouch in a mass of water on the outside, in order to dissipate the residual power of the corium by boiling of the mass of water.
The disadvantage of this device stems from the fact that refractory materials are usually very poor conductors of heat, the effect of this being to increase the equilibrium temperature of the corium which remains in a liquid state.
Other devices employing refractory floors cooled permanently by means of a water circuit are known. One of the disadvantages of these devices is that the cooling circuit can experience failures which are liable to make it at least partially ineffective, on the other hand, the heat exchanges are not sufficiently intense to prevent the corium from remaining at a high temperature and in the liquid state after it has spilled over onto the recovery and cooling device.
There is also a known device consisting of a stack of sections placed horizontally in the bottom of the well underneath the vessel bottom, so as to form receptacles for the melted corium, in order to accomplish the dispersal of the melted mass, improve its cooling and allow it to solidify. The disadvantage of this device, however, is that it does not afford effective protection of the concrete of this vessel well when the flow of corium occurs in a localized way. The sections which are in a staggered arrangement are then liable to fill in succession with melted corium as a result of local overflowing, in such a way that the melted mass can quickly reach the bottom of the vessel well.
Finally, French Patent Application No. 91-06047 discloses a device for recovering and cooling the melting core of a nuclear reactor, making it possible to avoid any contact between the mass of the melted core and the concrete of the vessel well and to ensure cooling and rapid solidification of the melted mass. This device consists of a metal structure resting on the bottom of the vessel well and immersed in a mass of water filling the lower part of the vessel well. The metal structure comprises a central shaft, a wall for recovering and cooling the melting core and a peripheral wall.
However, the disadvantage of this device, which makes it possible to carry out quickly a spreading, cooling and solidification of the corium, is that the metal structure can be destroyed under the effect of the dynamic forces generated during the fall of the corium and the vessel bottom, these forces being capable of reaching several thousand tons.
Moreover, the cooling of the corium gives rise to the formation of an extremely high vapor flow inside the vessel well, this flow being of the order of 10,000 m.sub.3 /hour. The elimination of such a vapor flow into the atmosphere of the containment brings about the dispersal of fission products in the entire volume of the reactor building, this being incompatible with the safety principles which must be adhered to.